If all you want to do is drive the car away, I'd take the rally team approach to body repair. Use a combination of zip ties and duct / elephant tape to temporarily reattach the bumper being careful not to obscure the headlight lens with the any tape.
I asked a police officer if I can remove the bumper and put a number plate in window and he said its illegal(is he right cos I've seen others do it).
I've seen others do that - it's not legal though, and you're asking for a fine of up to a £1000.
If so where can I mount the number plate and be legally able to drive in uk? Can I, and is it possible to ...
First, remove all the working parts of the subwoofer.
Most commercial boxes are only glued at the edges - it's cheaper if you use less glue. Try a hot air gun on a seam, with a thin taping knife to get underneath. If it starts coming away, continue. If it makes no difference, then you're going to either be cutting it off with a knife, or breaking the box ...
You intend to remove the bumper and use the car.
Alephzero's answer gives legislation governing the visibility of number plates (basically, vertical and clearly visible).
Your car must not pose an undue danger to other road users. Of course, any car hitting a pedestrian or cyclist is Not A Good Thing, but if the absence of a bumper exposes parts which will ...
You didn't specify what country, but since it looks like a RHD car I assume you mean the UK.
The regulations http://www.legislation.gov.uk/uksi/2001/561/part/II/made give the basic requirements as
It should be mounted vertically or as near as practically possible
It should be visible from any position in front of the car up to a distance of 22 meters, and ...
If you can peel up one corner then get yourself a paint scraper like this one:
and work your way between the box and carpet.
If you cannot peel up a corner then get a utility knife, pick a section to score, and then jam the paint scraper in there.
If this doesn't work then you ...